1. Field of the Invention
The present invention relates to a steering angle middle point detecting device for detecting a steering angle middle point of a steering wheel while a vehicle travels straight ahead, and more particularly concerns a power steering apparatus having such a steering angle middle point detecting device.
2. Description of the Background Art
For example, in a power steering apparatus which assists steering by using an electric-motor-driven hydraulic pump as a generation source of oil pressure, when a vehicle travels straight ahead with the steering wheel not being operated, an unloaded (no-load) current flowing through the electric motor has to be set in order to control the electric motor.
Moreover, in the case when there is an offset between an actual steering angle middle point and a steering angle middle point that has been set so as to provide control for imparting a steering assist force in accordance with the amount of variation of the steering angle from the steering angle middle point of the steering wheel while the vehicle travels straight ahead with the steering wheel not being operated, a compensating operation for the offset is carried out so as to prevent variations in the steering characteristic.
With respect to a device for detecting such a steering angle middle point, Japanese Patent Application Laid-Open No. 2-197465(1990) discloses a device, which is provided with a torque sensor for detecting a steering torque and a steering angle sensor for detecting a steering angle and which detects the steering angle middle point based upon a steering angle when the steering torque is smaller than a preset value, by utilizing the fact that the steering torque becomes smaller at the steering middle point.
Moreover, Japanese Patent Application Examined No. 6-43188(1994) discloses a method in which a steering angle sensor successively detects the steering angle and the steering middle point is specified based upon a statistical distribution of the frequency of occurrences of the detected steering angle data.
Furthermore, Japanese Patent Application Laid-Open No. 8-198129(1996) discloses a method in which: steering angle data is successively detected within a preset time based upon the origin that has been provisionally set, the variation width of the steering angle data is found and compared with a preset value, a hypothetical steering angle middle point is detected from the steering angle data that has been detected when the variation width is smaller than the preset value, and the steering angle middle point is detected based upon the frequency of appearances of a plurality of hypothetical steering angle middle points that have been time-sequentially found.
However, the above-mentioned conventional techniques have problems in which: a torque sensor is required to detect the steering angle middle point; a great deal of steering a angle data need to be sampled so as to obtain an accurate statistical distribution, with the result that much time and memory size for the data have to be provided; and sampling data are greatly varied depending on driving habits of drivers and road conditions.
Additionally, the above-mentioned problems may arise not only in so-called power steering apparatuses which carry out a steering assist by using an electric-motor-driven hydraulic pump as the generation source of oil pressure, but also in so-called electric power steering apparatuses which use an electric motor as the generation source of a steering assist force.